In the direct pressure type mold clamping devices for injection molding machines and die casting machines, it is necessary to open or close molds at a high speed for reducing the time of the molding cycle. In addition, it is also necessary to clamp molds with a large force to oppose the molding pressure.
To satisfy these opposite operations, this particular type of mold clamping device generally has a complex structure. Further, many kinds of mold clamping devices exist.
For example, a conventional mold clamping device for a plastic injection molding machine, which has been disclosed as Japanese Patent Provisional Publication gazette 57-115329, has the following structure.
Namely, a plurality of generally parallel liquid pressure driven cylinders for opening the mold and a high power clamping cylinder are provided. A single acting piston for mold clamping is fitted backward to a clamping ram of the clamping cylinder. The liquid pressure chambers of the liquid pressure driven cylinders are connected to the front oil chamber of the clamping cylinder. The front oil chamber and the rear oil chamber of the clamping cylinder are connected with each other by the connecting path of the piston section of the clamping ram. The connecting path is opened or closed by a first open-close valve which is operated by oil pressure from outside.
In this mold clamping device, at mold opening or mold closure, oil in the front oil chamber of the clamping cylinder and oil in the liquid pressure driven cylinders travel via the connecting path of the piston section of the clamping ram, so that negative pressure scarcely generates in the cylinders and smooth operation can be executed. Further, the clamping cylinder and the liquid pressure driven cylinders are arranged in parallel, there is an advantage of reducing total length of the device.
However, there are the following problems in the above stated mold clamping device.
(1) Because the mold opening operation is driven by the liquid pressure driven cylinders, a plurality of the liquid pressure driven cylinders must be provided on both sides of the clamping cylinder for balancing mold opening action.
(2) When the molds are opened, the oil chambers of the liquid pressure driven cylinders are pressurized so that the chambers are connected to the front and rear chambers of the clamping cylinder for oil flow. Therefore, the oil pressure affects not only the chambers of the liquid pressure driven cylinders but the front and rear chambers of the clamping cylinder so that the liquid pressure driven cylinders must have pressure proof structures. The front chamber of the clamping cylinder, which is not used for driving to open molds or to clamp them tightly, also must have a pressure proof structure. With such pressure proof structure, the device must be larger and heavier.
(3) The first open-close valve, which opens or closes the connecting path, provided in the piston section of the clamping ram is controlled in its operation by the oil path provided in the clamping ram and extended in the axial direction thereof and by the oil circuit connected to the path whose one end is opened to the surface of the clamping ram located in the vicinity of a movable board and is connected to a hydraulic machine via external piping. However, with this structure, it is difficult to machine or assemble the clamping ram, and there are problems in durability and safety because of the connecting of the external piping to movable portions.
(4) Many cylinder units, which need precision machining, are adopted so that manufacturing steps of producing the device are increased and the device will be expensive.